The present invention relates generally to internal combustion engines, and more particularly provides apparatus for reducing vibration in single cylinder engines typically utilized to drive portable rotary power tools such as flexible line trimmers, lawn edgers or brush cutters, and other portable rotary power tools such as snow throwers, shaft mounted blowers and the like, as well as single cylinder engines used in other applications.
In the design of single cylinder internal combustion engines of this general type, it is well known that essentially all of the rotating mass of the engine can be effectively balanced by appropriately configuring and sizing the crankshaft counterweight to thereby essentially eliminate the vibratory effects of such rotating mass. There are, however, forces within a single cylinder engine which cannot be overcome by the mass of the crankshaft counterweight. Such forces include the reciprocating mass of the piston and its associated connecting rod, and the forces required to suddenly accelerate and decelerate such reciprocating mass at the top dead center (TDC) and bottom dead center (BDC) positions of the piston. It is at these two positions where the single piston instantaneously comes to rest before changing directions in its reciprocable travel path. Adjacent the TDC and BDC positions of the piston, the piston and its associated connecting rod generate linearly directed inertia and reactive forces which are transmitted to the crankshaft in a direction transverse to its axis. These linear inertia and reactive forces tend to "rock" the crankshaft about its supporting bearing structure such that the connecting rod end of the crankshaft is moved in one direction and the flywheel end of the crankshaft "reacts" by moving in the opposite direction. This linear force-induced rocking of the crankshaft generates engine vibration which is not compensated for by the rotating counterweight portion of the crankshaft.
A previously proposed solution to this inertia-induced engine vibration has been to drill a small hole in the flywheel in an appropriate rotational orientation thereon, and then thread a screw, with an associated lock nut thereon, into the hole. The installed screw and associated nut act as a flywheel-mounted counterweight which, when the piston is adjacent its TDC or BDC position, exerts a force on the flywheel end of the crankshaft which is designed to offset the rocking moment imposed on the crankshaft by the piston and its connecting rod.
This conventional approach to reducing piston inertiainduced engine vibration, however, has distinct disadvantages from both safety and manufacturing standpoints. The primary disadvantage of this technique is that it imbalances the flywheel itself, thereby creating in the rapidly rotating flywheel a radially directed bursting force. It can readily be seen that this bursting force can potentially create a safety hazard associated with the previously carefully balanced flywheel.
Additionally, from a manufacturing standpoint, several additional fabrication steps are required in the production of the engine. The flywheel must be properly positioned so that the screw hole is precisely oriented thereon, the hole itself needs to be carefully formed in the correctly positioned flywheel, and the screw and its associated locknut must then be connected and carefully tightened into the flywheel hole. Moreover, of course, there is always the possibility that the screw and nut can work loose during engine operation and be thrown at high speed from the flywheel.
In view of the foregoing, it is an object of the present invention to provide improved apparatus for reducing inertiainduced vibration in a single cylinder engine of the type described to thereby eliminate or minimize above-mentioned and other problems, limitations and disadvantages typically associated with conventional solutions to this type of engine vibration.